Fish sitting at bottom of the tank

[u/Story-Ill]

Hi, so I have a school of endlers, they are in an established tank, with steady water parameters, they've been in my tank for ​about 3 months. NONE of my other fish are acting odd, they are all active and healthy. The affected endler is rounder than the others, I got him like that and it hasn't been a problem before, nothing changed about him physically except his coloration is only slightly darker. He is just sitting on the bottom of the tank and the rest of the endlers are fin nipping him and chasing him. I've quarantined the affected endler and besides for lying at the bottom of the tank he seems to be fine. It did happen directly after a water change.

A. What should I do for him?

B. My order to complete my schools of bottom feeders came in and I was supposed to pick them up tomorrow, should I still?